Ferrite core composition and method of preparation

ABSTRACT

FERRITE CORES, USEFUL OVER A WIDE TEMPERATURE RANGE, CONSISTING OF ABOUT 0.14 TO ABOUT 1.65 MOLE PERCENT TUNGSTIC TRIOXIDE, ABOUT 1 TO ABOUT 8 MOLE PERCENT MANGANESE OXIDE, ABOUT 11.35 TO ABOUT 17.86 MOLE PRECENT LITHIUM OXIDE AND ABOUT 75 TO ABOUT 86 MOLE PERCENT FERRIC OXIDE. THESE FERRITE CORES ARE MAGNETIC MATERIAL HAVEING A HIGH SQUARENESS RATIO THEREFORE HAVING A A BI-STABLE MAGNETIC PROPERTY USEFUL FOR STORING INFORMATION. A METHOD OF PREPARATION IS DESCRIBED WHEREIN THE CORE IS SINTERED FOR A PERIOD WHICH CAN BE AS SHORT AS ABOUT 10 TO 25 MINUTES.

United States Patent 3,575,862 FERRITE CORE COMPOSITION AND METHOD OFPREPARATION Earl J. Hoopes, Philadelphia, Pa., assignor to Fabri-TekIncorporated, Minnneapolis, Minn. No Drawing. Filed Mar. 18, 1969, Ser.No. 808,295

Int. Cl. C04b 35/26 US. Cl. 25262.61 16 Claims ABSTRACT OF THEDISCLOSURE Ferrite cores, useful over a wide temperature range,consisting of about 0.14 to about 1.65 mole percent tungstic trioxide,about 1 to about 8 mole percent manganese oxide, about 11.35 to about17.86 mole percent lithium oxide and about 75 to about 84 mole percentferric oxide. These ferrite cores are magnetic material having a highsquareness ratio therefore having a a bi-stable magnetic property usefulfor storing information. A method of preparation is described whereinthe core is sintered for a period which can be as short as about 10 to25 minutes.

SUMMARY OF THE INVENTION This invention relates to bi-stable ferritecores many types of which are generally well known in the art. Morespecifically, this invention relates to cores comprising a novelcomposition consisting essentially of tungstic trioxide (W0 manganeseoxide (MnO), lithium oxide (Li O) and ferric oxide (Fe O Those skilledin the art will recognize that many known ferrite cores have thedisadvantage of too narrow a temperature range over which they willexhibit stable operating characteristics. Also, those skilled in the artwill recognize the importance to production costs of the ability toprepare cores with a significantly decreased sintering time.

Cores prepared according to the novel composition and method describedherein are operable over a wide range of temperatures extending from 55C. to 125 C. Furthermore, and more important, these core exhibit stableoperating characteristics over any 75 C.l00 C. temperature range withinthe broad temperature range mentioned above without requiringcompensation.

Also, this invention is concerned with a method of preparing these coreswherein the sintering step can be accomplished in about 10 to 25 minutesor can extend over a period as long as about 8 hours.

DEFINITIONS The term squareness ratio commonly denoted R is definedherein as:

where B signifies the flux density in gauss, and H denotes the maximumfield strength (in oersteds) at the point of magnetic saturation.

Other terms used herein are defined as follows:

3,575,862 Patented Apr. 20, 1971 Symbol Name Definition Response voltageobtained at a ful read-current pulse, when preceded by a fullwrite-current pulse.

Response voltage obtained at a full read-current pulse preceded by aspecific number of partial read-current pulses.

Response voltage obtained at a full read-current pulse, when preceded bya full read-current pulse followed by a specified nuisnber of partialwrite-current pu es.

o Core peaking time Time between the 10% levelofreadcurrent pulse andthe peak of the V1 signal.

s Core switching time Time between 10% level of the readcurrent pulseand 10% level on the trailing edge of the uV1 signal.

Hu Coercive force The magnetomotive force per unit area necessary todemagnetize a core whic had previously been in the opposite state ofmagnetic saturation.

S.v Switching coetficienL... S..=t(H-H r 1 Read-disturbed 1 responsevoltage.

V: Disturbed zero response voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The relative proportions of theconstituents which produce the improved wide temperature cores inaccordance with this invention lie within the following ranges,expressed in approximate mole percentages.

W0 0.14-1.65 MnO 1-8 Li O 11.35-17.86 Fe O 84 A preferred range of molepercentages for the constituents is:

wo 0.22 0.7s MnO 1-s Li O 11.35-17.86 Fe O 75-84 The specific examplesillustrate the desirable properties of cores prepared in accordance withthis invention and having the following dimensions: outsidediameter-about 0.0230 inch, inside diameter-about 0.0150 inch,thicknessabout 0.0055 inch.

EXAMPLE Mole percent W0 0.7 MnO 2.6 Li O 16.4 Fe O 80.3

The components or initial constituents have ben listed herein as oxides.However, other compounds may be used initially in preparing the mixtureprior to the sintering or firing step. For example, hydroxides or morepreferably carbonates are generally used in preparing ferrite coresbecause they are easier to handle, are commercially available in highpurity form, and tend to increase the intimacy of the body upon burnoff. The following mixture uses carbonates as certain constituents:

EXAMPLE Mole percent 0.7

TABLE I Sintering time 8 hours 8 hours 8 hours. sintering temp 1,0651,065 O 1,074 Oxygen fiow 0.15 cu./ft./h1: 0.15 on. ft;./hr 0.15 on.ftJhr.

Core density (as 2.56 grn./ce 3.08 gm./cc 3.08 gin/cc. pressed).

Response and timing characteristics Other cores, prepared as indicatedin Table II below were found to possess similar desirable properties:

TABLE 11 Core density Response and Timing Characteristics Oxygen (assintering Sintering flow, pressed), Vt r dVz T s time temp., 0. cu.it.[hr gun/cc. (mv.) (mm) (mv.) (nsj (21s.)

1, 086 2. 68 32 28 9 123 275 1, 087 0. 5 2. 68 37 32 11 116 235 1, 0890. 5 2. 68 37 32 8 116 261 1, 088 0. 5 2. 68 36 32 8 129 271 088 0. 5 2.68 36 32 7 128 270 1, 099 0. 5 2. 68 32 25 11 121 268 1, 090 0. 5 2. 6843 31 11 123 251 1, 100 0. 5 2. 66 27 24 7 142 333 1, 087 0. 5 2. 56 3228 111 231 1, 085 0.5 3. 08 37 33 5 139 283 1, 086 0. 5 3. 08 43 37 6141 294 30 minutes- 1, 140 2. 5 3. O8 31 28 9 147 286 13 36 minutes- 1,110 3. 08 28 26 8 132 270 14 46 minutes 1, 120 20 3. 08 28 24 9 161 30315--- 8 hours 1, 080 0. 5 3. 08 32 28 9 123 260 16 o 1, 080 0. 6 3. 0833 7 138 283 -mesh screen. The mixture was then heated at a reactiontemperature of between about 650 C. and 950 C. for about 4 hours andthen milled in alcohol again for 24 hours with 50% additional millingmedia, after which it was pan dried and sieved once more.

After an additional period of drying, the batch was mixed with a resinbinder, for example about 3.8% by weight of Flexalyn 80M dissolved inmethyl ethyl ketone. This mixture was then sieved and dried again.

The screened reacted powder was then pressed into toroidal cores havingthe dimensions described above. The pressed cores were then sintered,preferably between about 1000" C. and 1300 C., in an oxidizingatmosphere, such as air, but preferably oxygen, and then removed fromthe sintering furnace after cooling to about 200 C., and finally wereair-quenched.

If the cores are sintered in air, an annealing or postsintering isnecessary at a reduced temperature. The annealing may be carried out inan inert atmosphere, such as nitrogen, argon, neon or helium containinga small amount of oxygen, i.e., about 1-3 volume percent, or in thealternative it may be carried out in a pure oxygen atmosphere. Sinteringin an oxygen atmosphere does not usually require the annealing step.

During sintering, the oxidizing atmosphere gas should be passed througha drying apparatus before passing into the sintering or firing furnaceand over the cores that are being fired. This step is necessary toprevent volatilization of the lithium component.

The data included in Table I is illustrative of the typical propertiesof cores prepared according to the invention under the conditionsindicated and which are similar in composition to the examples describedabove.

The values for the characteristics listed above in Table I were obtainedfrom a test set-up in which the testing conditions were as follows:

between points).

The same testing conditions produced the characteristics listed in Table11, except the drive current for Examples 6, 8 and 14 was 710 ma., andfor Example 12 was 755 ma.

For similar core properties but improved productthroughput perfurnace-hour, it has been discovered that the sintering step can bedrastically shortened contrary to the usual practice in this art.Examples 12, 13 and 14 above are illustrative of this discovery of ashort cycle hot insertion sintering method.

In the short cycle hot insertion sintering method the cores are loadedon platinum firing vehicles. These platinum firing vehicles are thenplaced end-to-end on a quartz firing carrier. The carrier and platinumtrays are mounted on a loading fixture positioned at the end of a muffiefurnace. The loading fixture may also be used to quench the platinumtrays and may therefore be equipped with a water-cooled cop er heatsink. The carrier is loaded into a pre-heated muflie to a predeterminedposition and allowed to remain at sintering temperature for about atleast 10 minutes. After about 10 minutes, the carrier is withdrawn toWhat was originally the loading position. The platinum trays may then bequenched on the Watercooled heat sink. A change in firing time willrequire an inversely proportional change in firing temperature; that is,if all other conditions are constant, a decrease in sintering time willnecessitate a proportional increase in sintering temperature. Therefore,in the short sintering method, the sintering temperatures are generallyhigher as illustrated by Examples 12, 13 and 14. Further test examplesof short time hot insertion sintering are shown in the following TableIII.

6 ing in amount from about 0.14 to about 1.65 mole percent for atungsten constituent about 1 to about 8 mole percent for a manganeseconstituent about 11.35 to about 17.86 for a lithium constituent,balance iron constituent comprising the steps of: mixing the initialtungsten, manganese, lithium and iron constituents together inpredetermined relative proportions; processing to standard ferromagneticceramic techniques; pressing at least a portion of the mixture into theshape of a core; and sintering the TABLE III Response and timingcharacteristics Oxygen sintering Sintering flow, cu. V1 V1 a Ts timetemp, C. it./hr. (n1v.) (mv.) (mv.) (ns.) (ns.)

1, 100 1 43. 37. 0 9. 6 128 277 l, 100 1 48. 0 40. 9. 9 125 275 1, 100 141. 5 36. 2 9. 5 127 286 1, 100 1 39. 9 35. 8 9. 7 135 290 1, 100 1 45.2 34. 7 11. 6 121 259 l, 100 1 37. 3 33. 5 9. 4 130 284 1, 100 1 40. 236. 0 8. 2 140 300 1, 100 1 38. 9 35. 3 8. 0 145 310 1, 100 1 36. 0 32.4 9. 5 131 285 do 1, 100 1 44. 2 35. 5 11.8 119 258 12 minutes 1, 150 147. 7 36. 4 14. 3 125 260 16 minutes 1, 130 1 40. 4 37. 5 9. 4 135 270do 1, 130 1 46. 7 42. 0 11. 3 125 255 22. 5 1, 120 1 34. 9 30. 6 9. 9125 265 minutes minutes 1, 120 1 33. 2 31. 3 9. 5 135 260 12 minutes 1,130 1 31. 7 29. 5 10. 8 130 250 Note-The values for the characteristitmlisted above in Table III were obtained from a test set-up in which thetesting conditions were as follows, with all drive current at a 0.61

The advantage of short sintering is that it reduces the furnace time perload from over a period of many hours to about 10 to minutes, therebyimproving manufacturing speed and efficiency. It is believed that suchshort sintering times have not been possible in the prior art as far assquare hysteresis loop, high speed lithium ferrites are concerned.

The features and embodiments described above will suggest manymodifications of the invention to those familiar with this art. It isaccordingly desired that this invention be limited only by the followingclaims.

What is claimed is:

1. A ferrite core consisting of about 0.14 to about 1.65 mole percenttungstic trioxide, about 1 to about 8 mole percent manganese oxide,about 11.35 to about 17.86 mole percent lithium oxide, balance ferricoxide.

2. The core of claim 1 wherein the ferric oxide is present in an amountranging from about 75 to about 84 mole percent.

3. The core of claim 2 wherein the tungstic trioxide is present in anamount ranging from about 0.22 to about 0.78 mole percent.

4. The core of claim 1 wherein the tungstic tiroxide is present in anamount of about 0.7 mole percent, the manganese oxide about 2.6 molepercent, the lithium oxide about 16.4 mole percent, balance ferricoxide.

5. The core of claim 4 wherein the ferric oxide is present in an amountof about 80.3 mole percent.

6. A method of preparing ferrite cores wherein the initial constituentsconsist of oxides or carbonates rangpressed core in an oxidizingatmosphere at a temperature of at least 1000 C. for a period of at leastabout 10 minutes.

7. The method of claim 6 wherein the mixture is pressed to a density ofabout 2.56 to about 3.08 gm./cc. prior to sintering.

8. The method of claim 6 wherein the sintering step is accomplished indry air at a temperature of between about 1000 C. and about 1300 C. fora period of about 8 hours, and sintering is followed by an annealingstep which is accomplished at a reduced temperature in an inertatmosphere containing a small amount of oxygen.

9. The method of claim 8 wherein the sintering temperature is betweenabout 1000 C. and about 1100 C.

10. The method of claim 8 wherein the cores are allowed to cool to'about 200 C. before removal from the sintering apparatus and then areair-quenched.

11. The method of claim 6 wherein the sintering step is accomplished indry oxygen at a temperature of between about 1000 C. and about 1300 C.for a period of about 8 hours.

12. The method of claim 11 wherein the cores are allowed to cool toabout 200 C. before removal from the sintering apparatus and then areair-quenched.

13. The method of claim 6 wherein the pressed cores are sintered forabout 10 to about 46 minutes at a temperature of at least about 1050 C.

14. The method of claim 13 wherein sintering is accomplished in dryoxygen.

8 15. The method of claim 13 wherein the pressed cores References Citedare placed in a pre-heated furnace and reach sintering UNITED STATESPATENTS temperature within about 2.5 minutes.

2,980,619 4/1961 Wetzel 25262.56 16. The method of claim 15 wherein thecoresare re- 3,223,641 12/1965 Lessofi et a1. 252 62 61 moved from thefurnace to a water cooled heat $111k and 5 3 372 123 3/1968 E SV 81 dtet a1 25Z 62 61 cooled in room atmosphere to room temperature withinabout 2 minutes. ROBERT D. EDMONDS, Primary Examiner

